Labyrinth seals are often used to reduce or control fluid leakage in systems having equipment such as gas and steam turbines, compressors, pumps, and other types of rotating equipment where fluid flow may occur between two relatively rotating members. More particularly, labyrinth seals are frequently used in sealing between elements such as a rotating shaft and an associated stator housing to inhibit undesirable fluid flow between the exterior of the shaft and interior of the stator. When incorporated with a pump, a labyrinth seal may be relied upon to inhibit leakage along the pump shaft of high pressure fluid being discharged by the pump.
A labyrinth seal is generally characterized by a series of cavities or grooves formed along the adjacent surfaces of two relatively rotatable members such as a rotor on a rotating shaft and a stator on an adjacent, fixed housing. These members generally define a partial barrier between areas of high and low fluid pressure. At successive stations along the length of the labyrinth seal, adjacent surfaces of the rotatable members are situated in close proximity to each other to define annular slit-like orifices. In many labyrinth seal designs, a series of cavities or chambers are formed at these stations in order to retard fluid flow through the seal to a desired level.
In operation, previously available labyrinth seal designs often form a fluid barrier between the rotatable members by forcing high velocity fluid to navigate irregularly spaced adjacent surfaces formed between the relatively rotatable members. The fluid is forced to pass sequentially through slit-like orifices to enter enlarged cavities where the fluid energy is largely dissipated in turbulence. One of the basic concepts of any labyrinth seal design is to create a highly frictional fluid flow path. Such a flow path will convert pressure energy into velocity energy. A large portion of the velocity energy will be dissipated as heat energy via turbulent action.
One source of turbulence is created as a result of wall shear friction between high velocity fluids and irregularly spaced adjacent surfaces of the seal. A second and often more important source of turbulence results from intense free shear layer friction between a high velocity fluid jet discharging from an orifice and relatively slow moving fluid in a large cavity immediately downstream from the orifice. As a result of the combination of these and other friction components, pressure energy is substantially reduce downstream of each orifice in a multi-cavity labyrinth seal system. The substantially reduced pressure in a given cavity formed downstream from a particular orifice results in smaller pressure changes occurring across additional downstream orifices. The net result is overall reduced leakage across the labyrinth seal.
A variety of labyrinth seal designs have evolved to take advantage of these principles of dissipating fluid energy. One early design is seen in U.S. Pat. No. 1,020,699--Kieser. In this design, a centrifugal pump is provided with a stepped and grooved sealing surface such that the kinetic energy of the fluid flow across the sealing surface is somewhat dissipated through designed turbulence.
In another such design, U.S. Pat. No. 1,482,031--Parsons, a labyrinth seal is characterized by a radially stepped surface provided along the rotor, the stator being provided with a corresponding set of barrier members or collars disposed in close relationship thereto. In this fashion, high pressure fluid moving across the sealing surface will encounter interference; thus, minimizing leakage. In yet another design, U.S. Pat. No. 3,940,153--Stocker, the labyrinth seal is characterized by a succession of annular orifices or clearances between sealing teeth or knives on one member, and generally cylindrical surfaces or lands on the other. In combination, the sealing system defines a doubly recurved flow path from each orifice to the orifice next downstream.
Such prior art systems employ the use of sharp turns in the fluid flow path to provide additional fluid friction or resistance to flow. The through-flow fluid is forced to "zig-zag" or "serpentine" through the seal. The turning of the through-flow fluid in the prior art is often achieved through the use of wall positioning and wall curvature. Many of the prior art configurations were designed without precise quantitative data and without fully appreciating the kinetics involved in turbulence generation and energy dissipation associated with a sealing system. The concern of the prior art has generally been to increase the wall shear friction through the use of long and tortuous flow paths between each pair of annular orifices. By focusing on the use of wall shear stress, the prior art often neglected the turbulence generating potential of a free (i.e. away from wall) shear layer.
This reliance on wall shearing to turn the through-flow fluid has caused prior art devices to be characterized by a variety of "knives" or "lands." These features have been combined either acutely or irregularly to increase the wall shear friction between the fluid and the sealing surfaces of the seal. This emphasis on wall shearing has resulted in prior art labyrinth seal systems having intricate and complex sealing surfaces.
The prior art has continued to employ wall shear friction to turn the through-flow fluid. More particularly, prior art devices have relied upon wall positioning and wall curvature to turn the fluid while failing to optimize the potential of fluid turning available in configurations capable of producing free shear layers.
This failure is particularly evident when prior art seals have been adapted for operation in harsh operating environments, such as military and space applications. To maintain an adequate seal in such harsh environments, a trend has been to increase the number of sealing cavities or chambers, as well as increasing the complexity of the relative geometry of adjacent sealing surfaces. Prior seals have also included the use of alreadable or honeycomb matrix materials attached to either the stationary or the rotating member of the seal. In a further effort to achieve more effective sealing, prior seals have increasingly restricted the relative clearance between the moving seal members by diverting a portion of the fluid through a system of intricately machined slots.
As a result of these trends in labyrinth seal design, many contemporary labyrinth seals have been quite expensive to produce due to the intricate machining required to finish each sealing surface. Such complex sealing surfaces have also been somewhat fragile and thus prone to failure in rigorous applications due to the inherent structural weakness of the thinly structured sealing teeth and associated buttresses. The increasingly small, tight tolerances required between the rotating members of such seals are often impractical when the rotary drive shaft is prone to move off center or wobble. Most important of these deficiencies, prior art sealing systems have generally failed to achieve optimum sealing efficiency in harsh operating environments.